Due to an explosive proliferation of a portable terminal market in recent years and improvements in the infrastructure associated therewith, more strict requirements have been made from the market for improvements on the efficiency of transmission amplifiers for base stations as well as transmission amplifiers for mobile stations. For this reason, attention has been focused in recent years on the trend of accomplishing a high-performance amplifiers by combining a technology of amplifying signals at high efficiency, as represented by the Doherty amplifier, with a technology of reducing distortions and a recent distortion compensation technology (see, for example, the following Documents 1-6).
Document 1: JP-2002-124840-A
Document 2: JP-7-22852-A
Document 3: JP-8-330873-A
Document 4: Published Japanese Translation of PCT International Publication for Patent Application No. 2001-518731
Document 5: Published Japanese Translation of PCT International Publication for Patent Application No. 2002-510927
Document 6: Published Japanese Translation of PCT International Publication for Patent Application No. 10-513631
The Doherty amplifier is a device intended to improve the efficiency of high-output power amplifier, and was first proposed in the following Document 7. The Doherty amplifier generally comprises a carrier amplifier and a peak amplifier which are implemented by devices having the same characteristics, and is composed of two to a plurality of them in parallel. A large number of Doherty amplifiers have been implemented as amplifiers which actually handle signals over frequency bands from low frequencies to millimeter waves.
Document 7: W. H. Doherty “A New High Efficiency Power Amplifier for Modulated Waves”, Proc. IRE, Vol. 24, No. 9, September. in 1936
The configuration of such a conventional Doherty amplifier is illustrated in FIG. 1. As illustrated in FIG. 1, this conventional Doherty amplifier comprises carrier amplifier 13 which is performing at all times a signal amplifying operation irrespective of an input signal level; peak amplifier 14 which performs an amplifying operation only during a high power output event in which an input signal level is equal to or higher than a certain level; output combiner circuit 15 for combining the output of carrier amplifier 13 with that of peak amplifier 14 to deliver the resulting output; and an input branching circuit 10 for distributing an input signal to a carrier amplifier 13 and to a peak amplifier 14. Peak amplifier 14 may be called an “auxiliary amplifier,” but the name “peak amplifier” is always used in this specification.
Generally, the Doherty amplifier has carrier amplifier 13 which operates while maintaining the saturation near the saturated output power to accomplish a higher efficiency than general class-A and class-AB amplifiers even when a backoff is removed from the saturated power for delivery. An amplifier biased to class AB or class B is often used as carrier amplifier 13. Also, peak amplifier 14 is often biased to class C in use such that it operates only when a high-power signal is generated.
Here, the backoff refers to the difference between average output power and saturated power, and a large backoff state shows a state in which the average output power is smaller as compared with the saturated power.
Output combiner circuit 15, which combines the output of carrier amplifier 13 with that of peak amplifier 14, is comprised of a transformer, and is generally implemented by a transmission line of one-quarter wavelength (¼ λ).
Input branching circuit 10 is comprised of a transmission line of one-quarter wavelength or a 90° hybrid circuit or the like for making a phase relationship between an output signal of peak amplifier 14 and an output signal of carrier amplifier 13 in phase at a signal combining point of output combiner circuit 15.
Since the operation principles of general Doherty amplifiers, specific examples of pre-distortion compensation circuits, and the like are well known to those skilled in the art, for example, from the following Document 8 and the like, their detailed configurations are omitted:
Document 8: Peter B. Kenington, “High-Linearity RF Power Amplifier Design”, P.351-420, P493-506, Artech House, 2000.
In the following, a description will be given of the operation of this conventional Doherty amplifier. To begin with, the operation of carrier amplifier 13 will be described. Generally, an amplifier suffers from an amplitude-amplitude distortion (hereinafter abbreviated as AM-AM) and amplitude-phase (hereinafter abbreviated as AM-PM) distortion as an operating point moves from a linear region to a saturation region, and causes a deviation (distortion) from a linear response, an example of which is shown in FIG. 2. This distortion appears more prominent as the operating point exceeds the saturation point of the amplifier and goes deeper into a compression region, and constitutes one cause for generating a distortion component such as cross-modulation, adjacent leak power and the like in a signal band and its vicinity.
Next, consider operating states of each of carrier amplifier 13 and peak amplifier 14 during the operation of the Doherty amplifier. Note that while a situation is assumed herein in which the same devices are used for both carrier amplifier 13 and peak amplifier 14 for simplicity, it is believed that the generality is not particularly lost.
The operation region of the Doherty amplifier is roughly divided into three operation regions: a low level region, a transition region, and a saturation region. In a region in which lower input power is applied to the Doherty amplifier (called a “low-level region”), peak amplifier 14 is biased to class C and is therefore cut off, so that it is not operating. On the other hand, carrier amplifier 13 is performing a normal amplifying operation. Then, as the input power gradually increases, carrier amplifier 13 reaches the saturation immediately before peak amplifier 14 starts the operation (this is called the “transition point”). At this time, the efficiency of the Doherty amplifier itself is maximized, and its efficiency increases to approximately 78% if carrier amplifier 13 is an ideal class-B amplifier. However, at this time, saturated output power of carrier amplifier 13 is still one quarter of saturated power which should be generated as the Doherty amplifier.
Further, as the input power increases, peak amplifier 14 now starts the operation, i.e., starts a signal amplifying operation as a class-C amplifier, and modulates a load impedance of carrier amplifier 13 through a transmission line transformer disposed in output combiner circuit 15. Carrier amplifier 13, while maintaining the saturated state, supplies larger power to a load which is modulated in accordance with the output power of peak amplifier 14. Thus, a linear amplification characteristic is maintained as the Doherty amplifier as a result, making it possible to generate desired saturated power. The amplifier efficiency is maintained extremely high from this transition point to the saturation point.
Next, the AM-PM distortion characteristic for the input level of the Doherty amplifier will be described with reference to FIG. 3. FIG. 3 schematically shows the AM-PM distortion characteristics of carrier amplifier 13, peak amplifier 14, and the overall Doherty amplifier with respect to the input level. In FIG. 3, curve A represents the AM-PM distortion characteristic of carrier amplifier 13; curve B represents the AM-PM distortion characteristic of peak amplifier 14; and curve C represents the AM-PM distortion characteristic of the Doherty amplifier.
In a transition region, carrier amplifier 13 performs an amplifying operation while maintaining the saturation, as described above, so that the AM-PM distortion becomes larger as the input level increases, as indicated by curve A in FIG. 3. However, as indicated by curve B in FIG. 3, peak amplifier 14 does not yet reach the saturation at this time, but is operating at an operating point at which a large backoff is present, so that its AM-PM distortion is relatively small. Also, since peak amplifier 14 does not significantly contribute to the output power of the Doherty amplifier, its AM-PM distortion can be almost ignored from the total AM-PM distortion characteristic. Note that the AM-PM distortions from the respective amplifiers need not be generated in different directions (signs) as in FIG. 3.
Next, considering a situation in which the input power is further increased so that the Doherty amplifier is generating saturated power, carrier amplifier 13 is maintaining the saturated state, and peak amplifier 14 also reaches the saturated state this time, so that the AM-PM distortion further increases as well in peak amplifier 14. It is therefore understood that the AM-PM distortions of carrier amplifier 13 and peak amplifier 14 differ in the level at which the distortion occurs, depending on the input power, i.e., the operating state of the Doherty amplifier.
While a high efficiency can be accomplished by the conventional Doherty amplifier as described above, a problem lies in that the AM-PM distortion grows as the input power is increased. A method generally contemplated for compensating such an amplifier for the AM-PM distortion characteristic may involve disposing a pre-distortion compensator at a stage previous to the amplifier.
As an amplifier which places emphasis on distortion, a plurality of identical amplifiers are often arranged in parallel to generate respective outputs which are combined to produce large power. In an amplifier in such a configuration, since each of the amplifiers is adjusted to a substantially similar characteristic, the respective amplifiers are basically identical in the total characteristics as well. A number of examples are found in documents and the like as to attempts to compensate such an amplifier for distortion by providing a common pre-distortion compensator for the amplifier. For example, as described in the following Document 9, such attempts are not uncommon. In this event, an almost satisfactory distortion compensation characteristic can be achieved for each of the amplifiers operated in parallel. Generally, however, they are operated while a predetermined backoff is ensured, so that they are limited to a low value in regard to the efficiency.
Document 9: N. Imai et. Al. “Novel Linearizer Using Balanced Circulators and Its Application to Multilevel Digital Radio Systems” IEEE Trans. Microwave Theory and Techniques Vol. 37, No. 8, August 1989.
A certain degree of distortion improving effects can be expected for the Doherty amplifier as well by using a pre-distortion compensation circuit which is configured to compensate the Doherty amplifier for the total AM-PM distortion characteristics just like the prior art. However, for improving the efficiency of the Doherty amplifier, since carrier amplifier 13 and peak amplifier 14 differ in the operation class, there is a difference in the level of generated AM-PM distortion depending on input power, i.e., an operating point and an operating state. For this reason, the uniform compensation relying on the pre-distortion compensation circuit, as the prior art, is not always the best solution. Specifically, when the Doherty amplifier is regarded as a single amplifier and a pre-distortion compensation is attempted therefor, an optimal AM-PM compensation is not always achieved in each of carrier amplifier 13 and peak amplifier 14, thus failing to provide optimal distortion compensation effects and ideal operations as the Doherty amplifier. Also, there is a problem in that a phase difference is not compensated for at an output combining point, thus failing to achieve an ideal power combining operation.
In other words, the conventional Doherty amplifier and associated distortion characteristic compensation method described above have a problem of the inability to reduce distortions such as the AM-PM distortion because the Doherty amplifier is composed of a carrier amplifier and a peak amplifier which differ in the operation class, though a high efficiency can be accomplished.